1. Field of the Invention
The present invention relates generally to systems and devices for electrooptically measuring and sensing voltages. More particularly, the present invention relates to systems and devices for sensing and measuring high voltages associated with electric fields produced by energized conductors.
2. Relevant Technology
The ability to accurately sense and measure power is an important aspect of power systems and the power industry. Currently, however, power measurement and metering is typically performed only when necessary, which frequently occurs on the high voltage or power source side before the voltage is stepped down for distribution. As the power industry deregulates, it is becoming more important to accurately track and measure power, which indicates that additional measuring and metering is needed in the power infrastructure. Power measurements are made by determining the values of both the current and the voltage. While current measurements are easily performed and are readily available as many current measurement devices are currently in place, voltage measurements are not readily available and can be rather difficult to accurately obtain.
High voltage measurement is traditionally accomplished using iron core ferromagnetic potential transformers. Potential transformers, however, are problematic for a variety of reasons. They exhibit a limited dynamic range, have limited bandwidth, and introduce a substantial degree of non-linearity. Also, potential transformers have been observed to unintentionally conduct dangerous levels of energy downstream towards equipment or personnel thereby creating a serious safety hazard.
Many conventional methods for sensing and measuring high voltages, including potential transformers, require direct electrical contact with the energized conductor, which has the major disadvantage of causing interruptions or interference with the power transmission of a system due to the presence of an additional load. Prior voltage sensing and measuring systems also tend to be relatively bulky due to the requirement for a large voltage divider which is necessary to connect the sensing element with the energized conductor. Large voltage dividers are not only space consuming, but are also expensive and difficult to implement in many situations. For example, the installation of a substation in a large city is quite difficult because the available real estate in which to install the substation is very limited. In other words, the ability to effectively measure high voltages as well as power can be difficult and expensive.
Another favored method for measuring a voltage or a potential is related to the electric field associated with the potential. Open air electric field sensors have been designed and built but are extremely susceptible to factors such as changes in ambient dielectric constant, adjacent conductor voltages, and conducting objects such as traveling motor vehicles, which induce signals and noise which can interfere with or override the reading of the voltage to be measured.
Relatively recently, optical sensors have been designed for voltage sensing applications, such as those which utilize interferometric modulation. Although relatively compact, such systems suffer from extreme temperature sensitivity, which makes these types of systems impractical for many situations. Optical sensors which are mechanically modulated have also been attempted, but suffer from unreliability due to failure of the moving parts in these systems.
Optical voltage sensors which operate by taking advantage of the Pockels effect have also been developed. The Pockels effect is an electro-optic effect which is manifested in certain crystalline materials which have the property of advancing or retarding the phase of polarized light waves when a voltage or an electric field is applied to the crystalline material. The effect on the phase of the light wave is linearly proportional to the first power of the applied voltage, which makes it ideal for accurate voltage sensing and measuring applications.
Thus, the electromagnetic beam or light wave passing through the Pockels crystal or cell undergoes an electro-optic effect when the crystal is subjected to an electric field. The electro-optic effect is observed as a differential phase shift, or a differential phase modulation, of the electromagnetic beam components in orthogonal planes of the electromagnetic radiation. The degree to which the electromagnetic beam is altered is indicative of the strength of the electric field. By determining the amount of the alteration, or the amount of the differential phase, the voltage being measured can be determined.
In order to effectively measure a high voltage, a typical Pockels cell voltage sensor requires a half-wave plate and a beam splitter. The beam splitter is used to separate the orthogonal components of the elliptically polarized beam such that the differential phase can be determined. The half-wave plate is necessary to properly orient the electromagnetic beam as it enters and leaves the Pockels crystal. If the half-wave plate is not positioned correctly, the voltage will not be measured accurately.
The half-wave plate is a both a critical part of current electro-optic voltage sensors and a source of inaccurate voltage measurements. The problem with half-wave plates stems from their extreme sensitivity to temperature. For example, the half-wave plate is responsible for rotating the electromagnetic beam by before it enters the Pockels cell. The wavelength of the light wave passing through the half-wave plate is very small, and the effect of temperature on the half-wave plate, which alters the dimensions of the half-wave plate, can ultimately have a significant effect on the accuracy of voltage measurements because the light wave is not properly rotated to the desired degree.
Another problem associated with many electro-optic voltage sensors is that they are inherently sensitive to temperature variations which introduce an intrinsic phase shift to the electromagnetic beam which is passing through the crystal. The reason for the intrinsic phase shift is related to the bi-refringent properties of the Pockels crystals. Many applications utilize a crystal in which the indices of refraction are not equal. As temperature changes, the indices of refraction also change. Unfortunately, the change in the indices of refraction is linear in each direction and the difference between the indices of refraction is thereby changing as the temperature changes. This change is both difficult to measure and ultimately remove from voltage calculations. As a result, an accurate measurement may require that the temperature of the Pockels crystal be constantly monitored. Otherwise, it is very difficult to account for the inherent differential phase shift which is thereby introduced. Additionally, monitoring the temperature introduces unwanted cost.
It would therefore be a significant advantage in the art to provide a compact voltage sensing or measuring apparatus which: does not require direct electrical contact with the energized conductor; is capable of accurate operation under a wide range of variable temperatures and environmental conditions; is reliable; is cost effective; and is substantially unaffected by temperature.